Fabrication of capacitive micromachined ultrasonic transducers by micro-stereolithography

ABSTRACT

An ultrasonic transducer is formed by a plurality of cMUT cells, each comprising a charged diaphragm plate capacitively opposing an oppositely charged base plate. The cMUT cells can be fabricated by conventional semiconductor processes and hence integrated with ancillary transducer circuitry such as a bias charge regulator. The cMUT cells can also be fabricated by micro-stereolithography whereby the cells can be formed using a variety of polymers and other materials.

This is a division of U.S. patent application Ser. No. 09/596,759 filedJun. 15, 2000.

This invention relates to medical ultrasonic imaging systems and, inparticular, to capacitive micromachined ultrasonic transducers for suchsystems.

The ultrasonic transducers used for medical imaging have numerouscharacteristics which lead to the production of high quality diagnosticimages. Among these are broad bandwidth and high sensitivity to lowlevel acoustic signals at ultrasonic frequencies. Conventionally thepiezoelectric materials which possess these characteristics and thushave been used for ultrasonic transducers have been made of PZT and PVDFmaterials, with PZT being the most preferred. However PZT transducersrequire ceramic manufacturing processes which are uniquely differentfrom the processing technologies used to manufacture the rest of anultrasound system, which are software and semiconductor intensive. Itwould be desirable from a manufacturing standpoint to be able tomanufacture ultrasonic transducers by the same processes used tofabricate the other parts of an ultrasound system.

Recent developments have led to the prospect that medical ultrasoundtransducers can be manufactured by semiconductor processes. Thesedevelopments have produced capacitive micromachined ultrasonictransducers or cMUTs. These transducers are tiny diaphragm-like deviceswith electrodes that convert the sound vibration of a receivedultrasound signal into a modulated capacitance. For transmission thecapacitive charge is modulated to vibrate the diaphragm of the deviceand thereby transmit a sound wave. Since these devices are manufacturedby semiconductor processes the devices have dimensions in the 10-200micron range. However, many such devices can be grouped together andoperated in unison as a single transducer element.

Since cMUTs are very small, it is desirable that constructed cMUTs haveas great a response to received acoustic signals as possible. A cMUTshould desirably exhibit as large a capacitive variation as possible toreceived signals. One approach to increasing the capacitive variation isto use electrodes only at the center of the cMUT diaphragm which willcause the capacitive charge to be located only at the center of themoving diaphragm. However, this arrangement requires the use of verysmall conductive paths to the electrodes, which increases the impedanceof these paths and thereby limits the response of the cMUT. It isdesirable to be able to increase the capacitive variation of a cMUTwithout the use of such high impedance conductive paths.

One of the advantages of cMUT transducers is that they can be made usingsemiconductor fabrication processes. Accordingly, cMUTs have beenfabricated using silicon and glass substrates for the base of thetransducers. These substrates form the back of the transducers oppositethe transmitting surface. Since transducers are intended to transmitmost of their energy out from the transmitting surface without radiatingappreciable acoustic energy out the back of the transducers or intoneighboring transducers through lateral coupling, a backing layer isusually applied to a transducer to damp or attenuate this undesiredacoustic energy. Accordingly it would be desirable to be able tofabricate cMUTs using materials which are better suited to reducing oreliminating this unwanted energy coupling.

cMUTs have been found to exhibit a response to applied transmit signalswhich is nonlinear due to the nonlinear electromechanical response ofthe charged cMUT diaphragm, which causes a corresponding quadraticsignal variation. Such a nonlinear response will result in distortion inthe transmit signal. This distortion can manifest itself as signalcomponents in the harmonic band of the desired transmit pulse, which canappear in the received echo signal as unwanted interference. It isdesirable to prevent such distortion from contaminating received echosignals.

A cMUT transducer is conventionally operated with a bias voltage whichcauses the transducer to have a range of operation which is notquadratic. The bias voltage must be carefully controlled so as tomaintain high transducer sensitivity without short-circuiting thetransducer's capacitance. It is desirable to be able to maintain theapplied bias in a condition which is stable in the presence of long-termeffects that can cause transducer short-circuits.

In accordance with the principles of the present invention a cMUTtransducer is described with improved signal response. The improvedresponse arises by reason of a nonplanar floor of the cMUT cell, whichconcentrates the cellular charge in the vicinity of that portion of thecell diaphragm which is most responsive to applied and received signals.A manufacturing process for cMUT transducers is described which enablesthe transducer to be fabricated by a technique ofmicro-stereolithography using polymeric materials. In operation the cMUTis biased by a controlled bias charge rather than a bias voltage. Thetransmission of unwanted signal components in the harmonic band isminimized by the use of predistorted transmit signals that counteractthe transducer's nonlinear response.

In the drawings:

FIG. 1 illustrates a cross-sectional view of a cMUT cell;

FIG. 2 illustrates the biasing of the diaphragm of a cMUT cell;

FIG. 3 illustrates the construction of a cMUT cell in accordance withthe principles of the present invention;

FIGS. 4a-4 j illustrate the fabrication of a cMUT transducer inaccordance with the principles of the present invention;

FIG. 5 illustrates the fabrication of a cMUT transducer bymicro-stereolithography;

FIGS. 6 and 6a illustrate the generation and maintenance of a biascharge for a cMUT transducer;

FIGS. 7a and 7 b illustrate the nonlinear characteristic of a cMUTtransducer;

FIGS. 8a and 8 b illustrate the effect of a nonlinear characteristic ona transmit waveform; and

FIGS. 9a and 9 b illustrate a transmit waveform resulting from the useof a predistorted transmit signal.

Referring first to FIG. 1, a typical cMUT transducer cell 10 is shown incross-section. The cMUT transducer cell 10 is fabricated along with aplurality of similar adjacent cells on a conductive substrate 12 such assilicon. A membrane or diaphragm 14 which may be made of silicon nitrideis supported above the substrate by an insulating support 16 which maybe made of silicon oxide or silicon nitride. The cavity 18 between thediaphragm and the substrate may be air or gas-filled or wholly orpartially evacuated. A conductive film or layer 20 such as gold forms anelectrode on the diaphragm, and a similar film or layer 22 forms anelectrode on the substrate. These two electrodes, separated by thedielectric cavity 18, form a capacitance. When an acoustic signal causesthe diaphragm to vibrate the variation in the capacitance can bedetected, thereby transducing the acoustic wave. Conversely, an applieda.c. signal will modulate the capacitance, causing the diaphragm to moveand thereby transmit an acoustic signal.

Due to the micron-size dimensions of a typical cMUT, numerous cMUT cellsare typically fabricated in close proximity to form a single transducerelement. The individual cells can have round, rectangular, or otherperipheral shapes. In a preferred embodiment the individual cells havehexagonal shapes to promote dense packing of the cMUT cells of atransducer element. The cMUT cells can have different dimensions so thatthe transducer element will have composite characteristics of thedifferent cell sizes, giving the transducer a broad band characteristic.

The cMUT is inherently a quadratic device so that the acoustic signal isnormally the harmonic of the applied signal, that is, the acousticsignal will be at twice the frequency of the applied electrical signalfrequency. To prevent this quadratic behavior a bias voltage is appliedto the two electrodes which causes the diaphragm to be attracted to thesubstrate by the resulting coulombic force. This is shown schematicallyin FIG. 2, where a DC bias voltage V_(B) is applied to a bias terminal24 and is coupled to the diaphragm electrode by a path which poses ahigh impedance Z to a.c. signals such as an inductive impedance. A.C.signals are capacitively coupled to and from the diaphragm electrodefrom a signal terminal 26. The positive charge on the diaphragm 14causes the diaphragm to distend as it is attracted to the negativecharge on the substrate 12. The device only weakly exhibits thequadratic behavior when operated continuously in this biased state.

It has been found that the cMUT is most sensitive when the diaphragm isdistended so that the two oppositely charged plates of the capacitivedevice are as close together as possible. A close proximity of the twoplates will cause a greater coupling between acoustic and electricalsignal energy by the cMUT. Thus it is desirable to increase the biasvoltage V_(B) until the dielectric spacing 32 between the diaphragm 14and substrate 12 is as small as can be maintained under operating signalconditions. In constructed embodiments this spacing can be on the orderof one micron or less. If the applied bias voltage is too great,however, the diaphragm can contact the substrate, short-circuiting thedevice as the two plates of the device are stuck together by VanderWalsforces. This sticking should be avoided when choosing a bias voltage forthe device.

Even when the diaphragm is biased to cause a very small sub-microndielectric spacing, the sensitivity of the cMUT can be less than thatwhich is desired. That is due to the fact that, whereas the charge atthe center of the diaphragm is relatively close to and will moveconsiderably in relation to the opposing charge, the charge at theperiphery 34 of the diaphragm where the diaphragm is supported by thesupport 16 will move very little and hence have little participation inthe transduction of signal by the device. One approach to eliminatingthis disparity has been to use a small diaphragm electrode which doesnot extend to the supports 16. This restricts the charge on thediaphragm electrode to the center of the device where it willparticipate strongly in the motion of the diaphragm and hence thetransduction by the device. There still must be one or more electricalconductors to apply the bias voltage to the diaphragm electrode and tocouple the a.c. signals to and from the electrode. These electricalconductors are necessarily very thin, with dimensions that imposeundesirably large impedances on the a.c. signals, thereby limiting thesensitivity of the device.

In accordance with the principles of one aspect of the presentinvention, the sensitivity of the cMUT device is improved without theuse of small electrodes or high impedance paths to such electrodes. Thisis done by increasing the ratio of the support spacing 44 to thedielectric spacing 42 by means of a nonplanar substrate. In theembodiment schematically shown in FIG. 3, the nonplanar substratecomprises a raised area or pedestal 28 on the substrate 12.Alternatively the nonplanar substrate can taper to a peak in the centerof the device. The diaphragm electrode will continue to cover the fulldiaphragm or be electrically connected by relatively wide, low impedanceconductors, thereby obviating the coupling problems of high impedanceconductors. Since the two electrodes are closely spaced in the center ofthe device, the charge density will be greatest in the center of thedevice as shown by the closely spaced positive charge in FIG. 3. Thecapacitive plates of the cMUT device are charged by a charge source 30coupled to the bias terminal 24. Thus, acoustic vibrations received bythe diaphragm will cause a relatively large modulation of the devicecapacitance and hence a relatively high degree of transducer coupling,providing a highly efficient cMUT device.

The nonplanar floor of the cell can be formed by starting with asubstrate with raised areas about which the cells are fabricated, or byleaving a raised center on the substrate when the cavity 18 between thediaphragm and substrate is formed, or by depositing raised areas duringfabrication of the cells. A process which uses the latter approach isshown in FIGS. 4a-4 j. Fabrication of the cMUT device starts with asilicon substrate 50 [FIG. 4a] which is treated to be highly conductiveand thus serve as one plate of a capacitance. A layer 52 of a stronglyconductive or high dielectric constant material is deposited on thesilicon substrate 50 [FIG. 4b]. This material preferably exhibits a highdielectric constant such as strontium titanate, or can be a refractorymetal that makes good contact with silicon such as gold or platinum ordoped silicon, for example. A photoresist film (not shown) is applied tothe surface of the conductive layer 52 and the resist film islithographically or E beam patterned. The conductive layer is thenetched to form raised areas 28 on the substrate and the resist isremoved [FIG. 4c]. Two oxide layers 54 a, 54 b are then laid over thesubstrate. The first oxide layer 54 a is thermally grown, and sincethermal oxidation consumes silicon the raised areas will be elevatedfurther by the process [FIG. 4d]. A second oxide layer 54 b is thendeposited over the first layer 54 a and the pedestals 28. The thicknessof this second layer 54 b determines the unbiased dielectric spacingbetween the diaphragm and the pedestal. Another resist film layer (notshown) is applied to the silicon dioxide layer 54 b, which islithographically or E beam patterned and etched to form circular,square, hexagonal or other shaped channels 56, separating the oxidelayers into shaped silicon dioxide islands 58 [FIG. 4e] and the resistis removed. This channeling process defines the shape or shapes of theindividual cMUT cells of the transducer. A silicon nitride film 60 isthen deposited over the silicon dioxide layers [FIG. 4f]. A resist filmis applied over the silicon nitride film and is lithographically or Ebeam patterned and etched to form apertures 62 which extend down to theupper silicon dioxide layer 54 b [FIG. 4g]. The resist is then removed.

The purpose of the apertures 62 is to expose the underlying silicondioxide film 54 to an etchant such as hydrofluoric acid which passesthrough the apertures and etches away the underlying oxide layers toform cavities 18 [FIG. 4h]. This leaves a silicon nitride diaphragm 60supported by silicon nitride supports 66 with a conductive pedestal 28below the diaphragm. The silicon nitride and the silicon substrate actas etch stops which define the size and shape of the cavity 18 of thecMUT cell. The final step is to form electrodes by applying a conductivefilm 70 to the upper surface of the silicon nitride diaphragm and aconductive film 72 to the lower surface of the substrate 50 [FIG. 4i].Prior to forming the conductive film 70 the structure may be subjectedto a further silicon nitride deposition which forms a layer 68 whichseals the apertures 62. The silicon nitride deposition can be carriedout under a vacuum so that the underlying cavity 18 may be at reducedpressure. Alternatively the apertures can be left open and the cMUTdiaphragm operated at atmospheric pressure. A plan view of a transducerelement of such cMUT devices is shown in FIG. 4j, in which the darklined hexagons 18′ define the cavities 18 of the devices, the cellsshare a common hexagonal patterned support 16, and the pedestals 28share the same hexagonal shape as the cells. A continuous electrode 70overlies all of the cMUT cells of FIG. 4j.

When the pedestal 28 is formed of a conductive material, the stickingproblem is reduced. This is because the charge which maintains thecapacitive plates in contact is quickly dissipated by the conductivematerial when the oppositely charged plates come into contact with eachother. On the other hand, the discharge of the bias charge will renderthe cell inoperative until the bias is reestablished. This situation isavoided by the use of a high dielectric material for the pedestal. Whilecontinuing to present the possibility of sticking, should the opposingplates touch only momentarily or bounce so that sticking does not occuror the VanderWals forces be overcome by the torsion of the diaphragmmaterial, the cell can continue to operate as the bias charge will notbe dissipated by the contact of the high dielectric pedestal with thediaphragm.

Unlike the prior approach, the electrode 70 is not etched to create highresistance conductive paths to small electrode areas over eachdiaphragm. The electrode layer can be formed as a continuous layercovering the diaphragms of a plurality of cMUT cells, or as individualelectrode areas each covering a majority of the diaphragm of a cell andpreferably extending out to the cell support. The individual electrodeareas are electrically connected to signal and/or bias circuitry by lowimpedance conductors, which may be formed of the same conductive layeras the electrodes. The low impedance electrodes and conductors provideefficient coupling to each capacitive cMUT cell on the wafer. Thepedestal 28 can extend several microns from the substrate floor of thecell. When the bias voltage V_(B) is applied to the device and thediaphragm is attracted to the pedestal 28, the spacing 42 between thediaphragm and pedestal can be on the order of approximately 0.25 μm,creating a high charge density at the center of the cell and providinggood sensitivity and coupling for applied signals.

While the cMUT cells and pedestals are both shown as hexagonal shaped inthe above example, different shapes for both can also be used. Thepedestals could be rounded (e.g., circular or elliptical), rectangular(square), or have other polygonal shapes. The pedestals can share thesame shape as the cavities defined by the support shape, or can havetheir own shape. A hexagonal cell with a circular pedestal is oneexample of this differentiation.

It is desirable that the electrical circuit formed by the cMUT device besimply a large variable capacitance. The use of an insulative materialsuch as silicon nitride for the diaphragm will effectively create asmall series capacitance in the cMUT circuit. This can be avoided byusing a highly conductive material for the diaphragm. One way toaccomplish this is to etch off the horizontal top layer of the siliconnitride film [FIG. 4f] by plasma etching, leaving the channels 56 filledwith insulating silicon nitride supports. A highly conductive materialsuch as doped polysilicon or a high strength refractory metal such asnickel or titanium is then deposited on the wafer to form the diaphragmmaterial. Thus, the layer 60 will comprise a conductive diaphragm layersupported above the oxide islands 58 by insulating silicon nitride, andthe finished cMUT cell will have a conductive diaphragm opposing thepedestal 28.

It will also be appreciated that the ratio of the support spacing 44 tothe dielectric spacing can also be improved by suspending the pedestalfrom the center of the underside of the diaphragm 14, opposing theplanar floor of the cell. That is, the diaphragm becomes a nonplanarstructure rather than the floor of the cell. This additional mass on thediaphragm will lower the frequency of the diaphragm and hence thefrequency of operation of the cMUT transducer, and will cause thevariability of the devices to be sensitive to the amount of materialused for the suspension, however.

Various conductive films and depositions can be used for the electrodesof the cMUT such as gold and aluminum. Instead of being applied to thelower surface of the substrate, the electrode 72 can be applied to theupper surface prior to deposition of the sacrificial layer 54.Polysilicon can also be a suitable material for the sacrificial layer.Other materials such as glass can be used for the substrate, in whichcase the substrate electrode is applied to the upper surface of thesubstrate. Details of the semiconductor processes and materials whichmay be used to construct an embodiment of the present invention aredescribed in U.S. Pat. Nos. 5,982,709 and 6,004,832 which areincorporated herein by reference.

Since the cells 10 can be fabricated by standard semiconductorprocesses, other associated electronics for the cMUTs can beconcurrently fabricated on the silicon substrate. Transmit or receiveelectronics for the transducer element such as amplifiers and highvoltage drivers can be fabricated on the same substrate as the cMUTcells.

An array of cMUT cells may also be fabricated by other processes such asmicro-stereolithography. In this process the cMUT-structures are builtup by depositing multiple layers ofmaterial through laser ablation. Anadvantage of this process is that cMUT cells can be built up on a widerange of substrates and using a wide range of cell constructionmaterials. Substrate materials which are more absorbent to ultrasoundthan silicon or glass, and hence form better acoustic backings to thecMUT cells, such as polymers, plastics and rubberized compounds can beemployed in such a process. These substrate materials can reduceunwanted acoustic coupling laterally through the substrate and out theback of the transducer. Shown in FIG. 5 is a polymer substrate 90 whichis more absorbent to ultrasound than silicon or glass. A conductive film(not shown) is applied to the upper surface of the substrate 90 to formthe lower electrode of the cMUT cell. The structure of the cMUT cell isthen built up layer by layer on the substrate. For example, a carrier 86carries a layer 84 of a material to be ablated for construction of thecMUT. For construction of the pedestal on the floor of the cell, thematerial 84 is a conductive material such as gold or aluminum. A laserbeam 80 is directed to the area of the carrier from which the material84 is to be deposited. The carrier 86 is transparent to the frequency ofthe laser beam so that the energy of the laser will ablate the materialon the side of the carrier facing the substrate. When an infrared lasersuch as a YAG laser is used, for instance, the carrier may be Teflon.Successive layers of conductive material 84 are ablated onto thesubstrate to deposit layers 92,94 of the pedestal until the pedestal isbuilt up to the desired height. The supports 96 for the diaphragm areinsulative and can be formed by ablating layers of a polymer or plasticin the desired locations. A film for the diaphragm is overlaid over thecells and can be welded onto the top surfaces of the supports 96 by thelaser. The thicknesses of the layers which can be deposited aredetermined by the laser power and the types of materials. For instance,metals, ceramics and plastics may be deposited in layers ofapproximately 1 μm or less. An ultraviolet wavelength laser such as anexcimer laser or a laser operated at a visible wavelength can also beused for this process, with a corresponding choice of the carriermaterial.

As mentioned previously a bias voltage V_(B) is applied to the cMUT toprevent quadratic operation of the device and to distend the diaphragmto a position close to the oppositely charged base of the device forgreater capacitive sensitivity. However, the capacitance of the cMUTcells can change over time, can differ from cell to cell on an array,and can differ from one array to another. As the capacitance changes thedevice draws more or less charge from the bias voltage source. This cancause the separation of the capacitive plates of the cMUT to change. Asmentioned above, if the base and diaphragm plates come into contact witheach other they can stick and render the cell inoperable. Furthermore,if the separation is not maintained at the optimum value, theperformance of the device will be degraded. In accordance with anotheraspect of the present invention, a bias charge rather than a biasvoltage is used to bias the cMUT cell. The bias charge can be applied bycoupling a current source to the cell for a known amount of time, forexample. A preferred embodiment of a bias charge source for a cMUT cellis shown schematically in FIG. 6. In this embodiment a current isselectively applied to the cMUT cell and the capacitance (orsusceptance) of the cell is measured to adaptively adjust and maintainthe bias charge of the cell. A field effect transistor 104 is regulatedto apply a bias charge to the diaphragm electrode 14. A small a.c.signal from a signal source 102 is applied to the cMUT cell. The smalla.c. signal may have a frequency of 10 kHz for instance. The resultinga.c. voltage produced on the cMUT device is sensed by a capacitancemeter or regulator 100. The sensed a.c. voltage is used to compute thecapacitance of the cMUT. The control signal applied to the field effecttransistor is adjusted in accordance with the sensed capacitance tomaintain or adjust the charge on the device and the process is repeateduntil the cMUT exhibits the desired capacitance and hence the properbias charge. The operation of the capacitance meter 100 is shown infurther detail in FIG. 6a. A small a.c. current i is applied to the cMUTcell (not shown) which is connected across terminals 106 and 108. Thequadrature component of the a.c. voltage developed across the cell bythe a.c. current is measured by a voltage meter v. The result of themeasurement is adaptively used to charge or discharge the capacitiveplates of the cell.

The cMUT capacitance is preferably periodically monitored by thecapacitance regulator 100 during use of the cMUT transducer. It will beappreciated that a bipolar gated circuit will permit the diaphragmelectrode 14 to be either charged and discharged as needed. In apreferred embodiment the bias charge circuit is constructed of elementswhich can be fabricated by semiconductor manufacturing processes and isintegrated onto the same wafer as the cMUT so that the cMUT cells andtheir bias charge source are integrally fabricated and co-located.

FIG. 7a shows a typical curve 110 illustrating the change indisplacement d of the diaphragm 14 of a cMUT as a function of theapplied bias charge or the resultant voltage V_(B). To avoid thequadratic behavior of the device which occurs around the origin of theplot, the bias voltage V_(B) shifts the nominal operating point 112 ofthe device to one side of the origin. In FIG. 7b the section of thecurve 110 around the biased operating point 112 is shown in greaterdetail. The curve 110 is shown tangential to a straight line 120, whichwould be a desired linear characteristic for the device. The curve 110is not linear, however. The lines denoted v_(l) and v_(h) mark the peakexcursions of an a.c. drive signal which is applied to the cMUT totransmit an acoustic pulse or wave. As the drawing shows, the nonlinearcurve 110 has a greater slope at the section 113 of curve 110 above thenominal operating point, and a lower slope at the section 111 below thenominal operating point 112. This means that the diaphragm will movenonlinearly when the cMUT is driven by transmit signal having peak topeak excursions of v_(l) and v_(h).

The effect of this nonlinearity may be appreciated by referring to FIGS.8a and 8 b.

FIG. 8a depicts a sinusoidal waveform 130 used as the drive signal for acMUT. This waveform is seen to have peak excursions of v_(h) and v_(l).When this waveform 130 is used to drive the nonlinear cMUT the resultingpressure wave 132 will have the characteristics depicted in FIG. 8b. Thefirst, positive half cycle will exhibit an overshoot above the desiredlevel of v_(h), and the second, negative half cycle will exhibit anundershoot below the desired level of v_(l) as the pressure waveform isdistorted by the nonlinearity of the device. In accordance with afurther aspect of the present invention the nonlinearity of the pressurewaveform is overcome by predistorting the applied drive signal 140 asshown in FIG. 9a. For transmission of a sinusoidal pressure wave thefirst half cycle undershoots the desired vh level and is more roundedthan the sinusoid, and the second half cycle overshoots the desired vllevel and exhibits a sharper peak than the sinusoid. The application ofthis drive signal 140 to the cMUT will produce the sinusoidal pressurewave 142 shown in FIG. 9b.

The reduction or elimination of this nonlinear effect is important whenthe cMUT transducer is used for harmonic operation. In both contrastharmonic and tissue harmonic operation it is desirable to transmit afundamental frequency waveform with minimal and preferably no spectralcomponents in the transmit signal's harmonic band. The only harmonicsignals sought during contrast harmonic operation are those returned bythe nonlinear effect of the contrast agent, and the only harmonicsignals sought during tissue harmonic operation are those produced, bydistortion of the pressure wave by the transmission medium. Transmittedsignal components in the harmonic band would contaminate these desiredsignals. Predistorting the drive waveforms to account for the nonlinearperformance of the cMUT devices will thus produce a transmitted pressurewaveform with substantially less nonlinear distortion from the cMUT andhence less artifact which would contaminate the harmonic spectrum of thetransmitted pulses.

What is claimed is:
 1. A method for fabricating a capacitivemicromachined ultrasonic transducer cell on a substrate, said cellcomprising a base plate which retains a first electrical charge, asupport, and a diaphragm plate which retains a second electrical chargecomprising depositing materials on said substrate to build up at leastone of said base plate, said support, and said diaphragm plate bymicro-stereolithography.
 2. The method of claim 1, wherein saidmicro-stereolithography comprises laser ablation.
 3. The method of claim2, wherein said laser ablation comprises ablating material from acarrier onto a substrate with a laser.
 4. The method of claim 3, whereinsaid carrier comprises a layer of a first material which issubstantially transparent at the frequency of said laser, and a layer ofa second material which is to be deposited onto said substrate.
 5. Themethod of claim 4, wherein said second material comprises one of ametal, a plastic, a polymer, and a ceramic.
 6. The method of claim 4,wherein said laser comprises an excimer laser.
 7. The method of claim 4,wherein said laser comprises a YAG laser.
 8. The method of claim 2,wherein said laser ablation comprises ablating multiple layers ofmultiple materials from one or more carriers onto a substrate with alaser.
 9. A capacitive micromachined ultrasonic transducer cellcomprising: a base plate which retains a first electrical charge; asupport having a top and a bottom, said bottom being located on saidbase plate and; a diaphragm plate which retains a second electricalcharge and is supported by the top of said support in a capacitiverelationship with said base plate; wherein said base plate includes asubstrate having an acoustic attenuation at an ultrasonic frequency ofinterest which is greater than the acoustic attenuation of silicon orglass.
 10. The capacitive micromachined ultrasonic transducer cell ofclaim 9, wherein said substrate is formed of a polymer.
 11. Thecapacitive micromachined ultrasonic transducer cell of claim 10, whereinsaid substrate comprises a plastic material.
 12. The capacitivemicromachined ultrasonic transducer cell of claim 9, wherein said cellis fabricated by micro-stereolithography.
 13. The capacitivemicromachined ultrasonic transducer cell of claim 12, wherein saidmicro-stereolithography comprises laser ablation.
 14. A method forfabricating a micromachined ultrasonic transducer cell on a substratecomprising: depositing multiple layers of materials at one or moreselected locations on said substrate to build up a support for adiaphragm on said substrate by micro-stereolithography; and overlaying adiaphragm over said support.
 15. The method of claim 14, wherein saidlayers are deposited by laser ablation.
 16. The method of claim 15,wherein said laser ablation uses an infrared or ultraviolet laser. 17.The method of claim 15, wherein said laser ablation comprises ablatingmaterial from a carrier onto a substrate with a laser.
 18. The method ofclaim 15, wherein said materials are deposited from one or more carrierswhich are substantially transparent at the frequency of said laser. 19.The method of claim 14, wherein said material comprise one or more of ametal, a plastic, a polymer, and a ceramic.
 20. The method of claim 14,wherein said substrate is comprised of a material having an acousticattenuation, at an ultrasonic frequency of interest which is greaterthan the acoustic attenuation of silicon or glass.